High quality flexo on corrugated

Flexo printing of corrugated or boxboard material to the high standards required by many customers is very demanding, writes Tom Kerchiss of RK Print Coat Instruments

In order to print high quality graphics on corrugated, converters have moved increasingly toward direct print which requires less workflow stages and less capital investment. For the flexographic printing specialist advances in technology and the development of more stable water-based flexo inks have addressed the issue of inferior quality once associated with direct print.

In order to run high quality process print using water-based flexo inks it is necessary to print very small, very sharp dots. Possible if the ink thickness printed on the material is reduced, the downside is a reduction in colour strength. For this reason, printers producing quality graphics demand high pigmented inks and then print them at very low film weights. There are several problems with using low film weight inks to minimise dot gain. If the paper of the corrugated board is rough there may be insufficient ink coverage so a compromise must be reached between dot gain and paper quality. Should the ink formulator raise the pigmentation level of the ink too much there will be insufficient resin to bind the pigment to the paper surface which also leads to a reduction in rub resistance.

Furthermore, the amount of ink printed onto the material is dependent on the viscosity of the ink – normally determined through consultation and experimentation via an ink supplier or press manufacturer. Once a decision has been made the printer must strive to maintain the determined viscosity level as variations in this area will result in changes in colour strength and print quality. Maintaining the desired viscosity level is easier said than done. Factors influencing viscosity include a combination of heat and the shearing action of the pumps. The latter may reduce viscosity by breaking down the small amount of thixotropy that exists in pigmented water-based flexo inks.

Ink manufacturers face challenges in terms of producing faster drying inks for today’s faster runs without compromising on re-solubility and wash up. Further issues include surface properties including rub and slip-resistance. As the surface of the box is covered with more and more graphics the co-efficient of friction or slip of the ink becomes ever more important.

Ink producers typically add waxes in order to improve scratch, scuff and rub resistance. This however alters the coefficient of friction and can cause problems when stacking and shipping, requiring a degree of compromise and consultation between printer/converter and ink supplier.

In order to control process variables as much as possible printers, converters, ink manufacturer, material producers and others use a variety of means to monitor quality and optimise their processes.

Designed and developed by RK Print Coat Instruments the Flexiproof helps resolve potential on-press and ink/substrate related problems off-press before they become costly on-press disasters. This compact benchtop system provides identical results under real operating conditions for paper, film and foil providers. Printability issues including gloss, scuff and chemical resistance, durability and flexibility can be determined. The FlexiProof enables users to resolve issues surrounding how the ink/substrate interact. Colour matching and further important aspects of colour management can be undertaken.

The multi-tasking nature of the device makes it suitable for trialling new jobs, freeing up the production press and providing savings in material costs, waste, further consumable costs – and time.

RK Print Coat Instruments

T: +44 (0)1763 852187